Maximising its political capital as the Prime Minister struggled to secure enough support from its backbenchers, the DUP made the decision to bail him out with just minutes to spare.

Without its support (315-306), which gave the government a nine-strong majority, the controversial plans would have been defeated.

Although both the party and Downing Street insist no deal was done it is understood that a series of pledges have been made giving Westminster support on issues at the top of the DUP agenda in a package worth more than £200m.

Over the last two weeks an increasingly desperate Mr Brown has showered the party with attention making almost daily phone calls and hosting a series of face-to-face meetings with key individuals.

He has targeted MPs he believed were most likely to take a tough stance on terrorism, particularly William McCrea and Nigel Dodds, as well as newly installed leader Peter Robinson.

Deputy leader Nigel Dodds said: "We have decided to vote for the extension of the pre-charge detention period after careful consideration of all the arguments.

"We believe, on balance, that this change in legislation is right and is in the best interests of national security in the United Kingdom.

"Over the last number of days we have had detailed discussions with the Government, opposition representatives, intelligence chiefs and other interested parties.

"As a result of our discussions it has become clear that there is a fundamental requirement for this measure, with all its parliamentary and judicial safeguards, to be in place so as our national security can be at an optimum level.

"Throughout our deliberations the security and safety of the citizens of the United Kingdom has been paramount.

Mr Dodds added: "Indeed, many of our MPs have, in the past, been the victims of terrorism therefore our decision is not based on party politicking but rather what is needed to safeguard our country and its citizens.

"It is clear that many in the House of Commons put the national interest above party politics on this particular matter.

"Our national security is much too important an issue to be at the behest of some horse-trading exercise. Our decision was based on what we felt was right for our country."

Last night, however, the DUP, which opposed 90-day detention, was accused of selling out its principles in return for Downing Street sweeteners.

Lib Dem leader Nick Clegg criticised the DUP for backing the government claiming: "I think principle has been lost."

But Mr McCrea said: "We made our decision on principle and we are proud to stand by it.

"We decided on an issue of principle on national security. The DUP and other Unionists voted side-by-side.

"Northern Ireland politicians know more than anybody in the UK what the ravages of terrorism are," he said.

The DUP and the UUP's lone MP Lady Sylvia Hermon, who also backed the plans, were given highly confidential briefings by security forces in the 24 hours before the vote setting out details of past and current terror investigations.